An X-ray CT is a powerful technique capable of acquiring an internal structure in a noninvasive and nondestructive manner and is being used for a wide range of medical purposes and industrial purposes such as defect inspection. The X-ray CT has been used for reconstructing an internal structure (CT image) based on the assumption that an object targeted for acquisition of its internal structure is at a standstill. However, objects targeted for X-ray CT for medical purposes include hearts, blood vessels, lungs and infants that find difficulty in stopping their motions completely. Applying an X-ray CT algorithm assuming a standstill to such active objects like in a conventional way is known to generate false images called motion artifacts.
Cardiac diseases have been requested to be dealt with in Japan and countries except Japan: cardiac diseases such as cardiac infarction and cardiac angina have been the second leading cause for deaths in Japan and heart diseases have been the leading cause for deaths in the U.S. in these 10 years. Finding lesions of hearts in an early stage is considered to be one of the most effective countermeasures. Accordingly, reconstructing accurate CT images has especially been desired in the field of X-ray CT on hearts.
A technique of reducing motion artifacts conventionally suggested has been targeted mainly for hearts in terms of significance of its application. As an example, according to known prior-art 1 (see non-patent literature 1, for example) shown in FIG. 14 in the field of X-ray CT targeted for hearts, an imaging target is scanned at high speed to shorten imaging time, thereby reducing motion artifacts. According to known prior-art 2 (see non-patent literatures 2-4, for example) shown in FIG. 15, a CT image is reconstructed by synchronizing a cardiac phase based on an electrocardiogram. According to known prior-art 3 (see non-patent literatures 5-9, for example), a CT image is reconstructed by correcting a motion based on estimation of the motion of a heart.
A more intense X-ray leads to a higher SN ratio of a reconstructed image, resulting in a trade-off between reduction in a dose of exposure with an X-ray and the SN ratio. Techniques of acquiring a reconstructed image having an SN ratio substantially the same as conventional one with a lower dose of exposure with an X-ray include: a technique intended to enhance a resolution capable of acquiring an image of a higher resolution with an X-ray intensity and the number of captured images same as a conventional intensity and a conventional number; a technique intended to shorten imaging time capable of shortening imaging time by reducing the number of captured images with an X-ray intensity and a resolution same as a conventional intensity and a conventional resolution; a technique intended to reduce an exposure dose capable of reducing an exposure dose by reducing an X-ray intensity with a resolution same as a conventional resolution; a technique intended to remove noise capable of reducing noise in a reconstructed image with an X-ray intensity and a resolution same as a conventional intensity and a conventional resolution; a technique intended to enhance a contrast resolving power (indicating the number of applicable tones of a reconstructed image, specifically the accuracy of X-ray absorption coefficients) of the reconstructed image with an X-ray intensity and a resolution same as a conventional intensity and a conventional resolution.
The aforementioned conventional techniques are roughly divided into two: one that makes statistical estimation and one that does not perform statistical estimation. A filtered back projection (FBP; Filtered Back Projection) method forming the mainstream of an image reconstruction method is known as the conventional technique that does not perform statistical estimation. Projection images are obtained through the irradiation of X-ray to the imaging target from various directions.
According to the FBP image reconstruction method, the resultant projection images are projected in reverse directions to the irradiation direction in order to reconstruct an X-ray absorption image. In this reconstruction, the sinogram is filtered in a frequency range to remove blur.
Where multiple projection images such as CT images can be prepared by capturing images of an object targeted for imaging from different positions or angles, processing information about these projection images is known to enable reconstruction of a cross-sectional image of the original object. A MAP (maximum a posteriori) estimation method and a Bayesian estimation method are known as conventional techniques to perform statistical estimation belonging to image reconstruction methods of restoring a cross-sectional image of an original object using a projection image.
[Non-patent literature 1] H. I. Goldberg et al., “Evaluation of ultrafast CT scanning of the adult abdomen,” Invest. Radiol., 24, 537-543, 1989.
[Non-patent literature 2] C. C. Morehouse et al., “Gated cardiac computed tomography with a motion phantom,” Radiology 134, 134-137, 1980.
[Non-patent literature 3] P. M. Joseph and J. Whitley, “Experimental simulation evaluation of ECG-gated heart scans with a small number of views,” Med. Phys. 10, 444-449, 1983.
[Non-patent literature 4] B. Ohnesorge et al., “Cardiac imaging by means of electrocardiographically gated multisection spiral CT: initial experience,” Radiology 217, 564-571, 2000.
[Non-patent literature 5] C. J. Ritchie et al., “Correction of computed tomography motion artifacts using pixel specific back-projection,” IEEE Trans. Med. Imaging 15, 333-342, 1996.
[Non-patent literature 6] G. Wang et al., “A knowledge-based cone-beam x-ray CT algorithm for dynamic volumetric cardiac imaging,” Med Phys. 29(8), 1807-1822, 2002.
[Non-patent literature 7] K. Taguchi et al., “Toward time resolved cardiac CT images with patient dose reduction: image-based motion estimation”, Nuclear Science Symposium Conference Record, 4, 2029-2032, 2006.
[Non-patent literature 8] AA. Isola et al., “Fully automatic nonrigid registration-based local motion estimation for motion-corrected iterative cardiac CT reconstruction,” Med Phys. 37(3), 1093-1109 2010.
[Non-patent literature 9] AA. Isola et al., “Motion compensated iterative reconstruction of a region of interest in cardiac cone-beam CT” Comput. Med. Imag. Grap. 34(2), 149-159 2010.